Introduction
A customer switches off the inverter and solar charger.
No external load is connected.
No charger is operating.
However, after two LiFePO4 batteries are connected in parallel, the BMS shows:
- Battery A: -18A
- Battery B: +18A
One battery appears to be discharging while the other is charging.
This often leads to the question:
“Why are my parallel LiFePO4 batteries charging each other when nothing else is connected?”
In many cases, this is caused by equalization current.
When two batteries with different terminal voltages are directly connected in parallel, the higher-voltage battery naturally sends current into the lower-voltage battery until their voltages move closer together.
A small temporary equalization current may be expected under controlled conditions.
A very high current, persistent current or repeated battery-to-battery current can indicate an installation or battery-matching problem.
1. Why Does Current Flow Between Parallel Batteries?
When batteries are connected in parallel:
- Positive connects to positive
- Negative connects to negative
Their terminal voltages are forced toward the same electrical potential.
Suppose:
Battery A
Voltage: 53.4V
Battery B
Voltage: 51.8V
The difference is:
1.6V
When the two batteries are connected directly, Battery A sees Battery B as a lower-voltage electrical path.
Current therefore flows:
Battery A → Battery B
until the voltage difference becomes smaller.
This is similar to connecting two water tanks at different water levels.
Water naturally flows from the higher level toward the lower level.
2. Why LiFePO4 Batteries Can Produce High Equalization Current
The problem is that LiFePO4 batteries have relatively low internal resistance.
The basic relationship is:
Current ≈ Voltage Difference ÷ Total Resistance
Assume the total resistance of the complete current path between two batteries is approximately:
0.01Ω
If their voltage difference is:
1V
the theoretical initial current could be:
1V ÷ 0.01Ω = 100A
Real systems are more complicated because resistance and BMS behaviour change dynamically, but the example illustrates an important point:
Even what appears to be a small voltage difference can produce substantial current.
This is why battery voltage should be checked before parallel connection.
3. Is Equalization Current the Same as Charging Current?
Electrically, one battery is indeed charging the other.
However, it is different from controlled charging by an inverter or battery charger.
A normal charger controls parameters such as:
- Maximum current
- Maximum voltage
- Charge stage
- Temperature
- BMS communication
Direct battery-to-battery equalization current may initially be limited mainly by:
- Battery internal resistance
- BMS resistance
- Cable resistance
- Fuse resistance
- Terminal resistance
There may be no external device actively controlling the current.
This is why directly connecting batteries with a large voltage difference should be avoided.
4. What Is Circulating Current?
The terms equalization current and circulating current are sometimes used interchangeably, but they can describe slightly different situations.
Equalization Current
Usually occurs after batteries with different voltages are connected together.
Current flows temporarily until their voltages become more similar.
Circulating Current
Can describe current that continues flowing between parallel sources even though there is little or no external load.
In battery systems, persistent circulating current may be associated with:
- Voltage measurement differences
- Different SOC
- Different BMS behaviour
- Internal resistance mismatch
- Different battery temperatures
- BMS switching or recovery events
A short initial current pulse can be normal.
Persistent high current should be investigated.
5. Why SOC Difference Can Create Battery-to-Battery Current
Suppose two identical 51.2V 100Ah batteries show:
- Battery A: 90% SOC
- Battery B: 30% SOC
Even if both are nominally “51.2V batteries,” their actual terminal voltages may differ.
Connecting them directly can cause Battery A to transfer energy into Battery B.
The user may think:
“The BMS will automatically balance them.”
The BMS provides protection, but it should not be treated as a replacement for correct pre-connection preparation.
A large equalization current may:
- Trigger overcurrent protection
- Trip a branch breaker
- Cause connector arcing
- Heat cables
- Stress BMS MOSFETs or contactors
6. Why Two Batteries Can Show Similar SOC but Different Voltage
SOC display itself is not sufficient.
For example:
- Battery A: 70% SOC, 52.8V
- Battery B: 70% SOC, 52.2V
Both display 70%.
Yet there is still a voltage difference.
Possible reasons include:
- Different SOC calibration
- Different resting time
- Different battery temperatures
- Different cell balance
- Recent charging or discharging
- BMS measurement variation
Before paralleling batteries, compare actual voltage, not only the SOC percentage.
7. Surface Charge Can Cause Misleading Voltage
A battery that has just finished charging may temporarily show a higher open-circuit voltage.
For example:
Battery A was recently charged.
Battery B has been resting for several hours.
Even if their actual stored energy is similar, Battery A may initially display a higher voltage.
Immediately connecting them can cause a short equalization current.
Allowing batteries to rest before voltage comparison can sometimes provide a more meaningful reading.
The exact commissioning procedure should follow the battery manufacturer’s instructions.
8. What Happens If One BMS Is Off?
This depends on BMS design.
Some batteries isolate the main terminals when switched off.
Others may still have:
- Pre-charge path
- Detection circuit
- Charging recovery path
If one battery is connected to an energized parallel bus while its BMS is off, the bus voltage may cause the battery to wake or attempt recovery.
The behaviour is model-specific.
This is why parallel systems should follow a defined startup sequence rather than randomly switching battery modules on and off.
9. Why Arcing Can Occur During Parallel Connection
When two batteries have different voltages, the moment the final connection is made, high current may flow.
This can cause a visible spark or arc.
Another source of connection spark is charging capacitors inside:
- Inverters
- DC-DC converters
- Controllers
But if two batteries alone produce significant arcing, voltage mismatch should be investigated.
Repeatedly connecting and disconnecting high-current DC circuits can damage:
- Terminal surfaces
- Connectors
- Breakers
- Cable lugs
and creates a safety risk.
10. Example: 51.2V 100Ah Batteries
Suppose:
Battery A
53.0V
Battery B
52.9V
Difference:
0.1V
This may produce relatively modest equalization depending on the complete system resistance.
Now compare:
Battery A
54.0V
Battery B
51.5V
Difference:
2.5V
The potential equalization current can be much larger.
The important point is that voltage difference should not be judged only as:
“2.5V is small compared with 51.2V.”
For a low-resistance battery system, 2.5V can be electrically significant.
11. Why One Battery May Repeatedly Charge Another
If battery-to-battery current continues repeatedly during normal operation, investigate:
Different SOC Calibration
One BMS may think it is fuller than the other.
Different Usable Capacity
An older battery may reach upper or lower limits sooner.
Different Internal Resistance
Current may redistribute after external load disappears.
Different BMS Protection Thresholds
One battery may disconnect and reconnect repeatedly.
Different Charge Cut-Off Behaviour
One module may stop accepting charge before the others.
12. What Happens When the Inverter Load Suddenly Stops?
Suppose two parallel batteries are supplying:
- Battery A: 60A
- Battery B: 40A
The inverter suddenly turns off.
Current from both batteries drops toward zero.
Because Battery A and Battery B may now have slightly different terminal voltages after different current loads, a small transient current can flow between them.
This may appear on the BMS app as:
- +2A
- -2A
for a short period.
Small brief values are not automatically abnormal.
A persistent 20A, 30A or 50A transfer with no external load deserves investigation.
13. Can Different Cable Resistance Cause Circulating Current?
Cable resistance mainly influences load sharing.
However, unequal current sharing can indirectly create different:
- SOC
- Voltage recovery
- Temperature
between batteries.
After a high load stops, those differences may produce temporary battery-to-battery current.
Therefore, balanced cabling remains important even when the main complaint is circulating current.
14. Why One Battery May Be Charging While Solar Is Also Charging
Suppose solar charging is providing 80A total.
BMS data shows:
- Battery A: +55A
- Battery B: +25A
This does not mean Battery A is charging Battery B.
Both batteries are receiving external charging current, but the current is distributed unevenly.
To determine whether actual battery-to-battery current exists, observe the bank with:
- External charger off
- Solar off
- Inverter load off
according to safe system procedures.
If one battery still reports positive current while another reports an approximately corresponding negative current, inter-battery transfer is more likely.
15. How Much Voltage Difference Is Acceptable?
There is no universal voltage difference that applies to every battery model.
Acceptable pre-parallel voltage depends on:
- Battery voltage
- Capacity
- Internal resistance
- BMS design
- Manufacturer procedure
Do not use an arbitrary number from another battery brand.
The safest approach is:
Follow the specific battery manufacturer’s required voltage and SOC matching procedure.
For professional ESS installations, modules should normally be prepared to closely matched operating conditions before being joined to the common bus.
16. Should You Use a Resistor to Pre-Balance Batteries?
For some professionally designed systems, controlled pre-charge or equalization methods may be used.
However, a resistor should not be improvised without calculating:
- Resistance
- Current
- Power dissipation
- Voltage
- Connection time
- Thermal load
A resistor that is too small may still allow excessive current.
A resistor that is underrated can overheat rapidly.
For commercial systems, use the manufacturer’s approved commissioning procedure.
17. A Better Way to Add a Battery to an Existing Bank
Before adding a battery:
Step 1
Isolate the new battery according to approved procedures.
Step 2
Check for BMS alarms.
Step 3
Measure actual battery voltage.
Step 4
Compare SOC with the existing bank.
Step 5
Charge or discharge the new battery as required to bring it close to the existing bank condition.
Step 6
Switch off the complete system according to the manufacturer’s procedure.
Step 7
Complete the parallel power connection.
Step 8
Connect communication wiring and configure battery addresses.
Step 9
Restart with a controlled load.
Step 10
Check individual branch currents.
18. Why Hot-Plugging a New Battery Is Risky
Imagine an existing battery bank is operating at:
54V
A new battery has been sitting in storage at:
50.5V
Connecting the new battery directly to the live bus can create a large current from the existing bank into the new battery.
This can happen before the inverter even begins using the new module.
Potential results include:
- Arcing
- Breaker trip
- BMS protection
- Connector damage
- Sudden communication alarm
Hot expansion should only be performed when the entire battery system is specifically designed for it.
19. Why the Existing Batteries May All Charge the New Battery
Suppose four existing batteries are paralleled and a fifth lower-voltage battery is added.
Energy can flow from all four original modules toward the new one.
If each old battery contributes 20A, the new battery could briefly receive approximately:
80A
even though no charger is running.
Therefore, the equalization current seen by the new battery can be much larger than the current from any one existing module.
This is an important consideration when expanding large banks.
20. Troubleshooting Persistent Battery-to-Battery Current
If current continues with no external source or load:
- Confirm measurement accuracy
- Verify no hidden DC load is operating
- Disconnect charging sources according to safe procedures
- Compare battery voltage
- Compare SOC
- Compare cell voltages
- Check BMS alarm history
- Compare temperature
- Inspect branch resistance and cable layout
- Check whether one BMS is repeatedly disconnecting and recovering
The BMS log can be particularly useful.
Frequently Asked Questions
Why does one LiFePO4 battery charge another in parallel?
Usually because the two batteries have different terminal voltages.
Is battery-to-battery current normal?
A small temporary equalization current can occur. Large or persistent current should be investigated.
Can two batteries with different SOC be connected in parallel?
They should be prepared according to the manufacturer’s approved voltage/SOC matching procedure before connection.
Why does the BMS trip when I connect the second battery?
The voltage difference may be causing a high equalization current.
Can parallel batteries exchange current even when the inverter is off?
Yes, if their terminal voltages differ.
Does the BMS prevent all equalization current?
No. The BMS may protect against excessive current, but it does not eliminate the electrical tendency for parallel batteries to equalize voltage.
Conclusion
Parallel-connected LiFePO4 batteries do not simply sit beside each other independently.
Because they share the same DC bus, a voltage difference between modules can cause energy to flow from one battery into another.
Temporary equalization may occur because of:
- Different SOC
- Recent charge history
- Different internal resistance
- Battery temperature
- BMS behaviour
Large uncontrolled equalization current should be avoided.
The safest approach is to prepare batteries to closely matched operating conditions before parallel connection and commission the system using a controlled startup procedure.
For installers and distributors, understanding battery-to-battery current is especially important when adding new battery modules to existing energy-storage systems.
HIZN Lithium supplies modular LiFePO4 battery solutions for residential solar, off-grid, telecom, UPS and commercial energy-storage systems, including CAN/RS485 communication and expandable parallel configurations.